1. Field of the Invention
This invention relates to light emitting diodes (LEDs) and more particularly to new structures for enhancing the extraction of light from LEDs.
2. Description of Related Art
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted omnidirectionally from the active layer and from all surfaces of the LED.
There has been a great deal of recent interest in LEDs formed of Group-III nitride based material systems because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The doped and active layers are typically formed on a substrate that can be made of different materials such as silicon (Si), silicon carbide (SiC), and sapphire (Al2O3). SiC wafers are often preferred because they have a much closer crystal lattice match to Group-III nitrides, which results in Group III nitride films of higher quality. SiC also has a very high thermal conductivity so that the total output power of Group III nitride devices on SiC is not limited by the thermal resistance of the wafer (as is the case with some devices formed on sapphire or Si). Also, the availability of semi insulating SiC wafers provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible. SiC substrates are available from Cree Inc., of Durham, N.C. and methods for producing them are set forth in the scientific literature as well as in U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022.
The efficient extraction of light from LEDs is a major concern in the fabrication of high efficiency LEDs. For conventional LEDs with a single out-coupling surface, the external quantum efficiency is limited by total internal reflection (TIR) of light from the LED's emission region that passes through the substrate. TIR can be caused by the large difference in the refractive index between the LED's semiconductor and surrounding ambient. LEDs with SiC substrates have relatively low light extraction efficiencies because the high index of refraction of SiC (approximately 2.7) compared to the index of refraction for the surrounding material, such as epoxy (approximately 1.5). This difference results in a small escape cone from which light rays from the active area can transmit from the SiC substrate into the epoxy and ultimately escape from the LED package.
Different approaches have been developed to reduce TIR and improve overall light extraction, with one of the more popular being surface texturing. Surface texturing increases the light's escape probability by providing a varying surface that allows photons multiple opportunities to find an escape cone. Light that does not find an escape cone continues to experience TIR, and reflects off the textured surface at different angles until it finds an escape cone. The benefits of surface texturing have been discussed in several articles. [See Windisch et al., Impact of Texture-Enhanced Transmission on High-Efficiency Surface Textured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15, October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl. Phys. Lett., Vol 64, No. 16, October 1993, Pgs. 2174-2176; Windisch et al. Light Extraction Mechanisms in High-Efficiency Surface Textured Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel et al. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. March/April 2002].
U.S. Pat. No. 6,410,942, assigned to. Cree Inc., discloses an LED structure that includes an array of electrically interconnected micro LEDs formed between first and second spreading layers. When a bias is applied across the spreaders, the micro LEDs emit light. Light from each of the micro LEDs reaches a surface after traveling only a short distance, thereby reducing TIR.
U.S. Pat. No. 6,657,236, also assigned to Cree Inc., discloses structures for enhancing light extraction in LEDs through the use of internal and external optical elements formed in an array. The optical elements have many different shapes, such as hemispheres and pyramids, and may be located on the surface of, or within, various layers of the LED. The elements provide surfaces from which light refracts or scatters.